In a population of beetles, some are green, some are brown, and some are mixed. if the beetles live in vegetation, the colored beetle is "least fit" and least likely to survive in that habitat when birds come along looking for food.

This could go both ways. Green beetles would do well if the vegetation is green, brown beetles would do well if the vegetation is brown, and mixed beetles would do well when the vegetation is a mixture of green and brown.

I think the question is implying that the vegetation is green, so brown beetles would be least likely to survive.

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products

after a the power stroke, adp is released, but the cross-bridge formed is still in place. atp then binds to myosin, moving the myosin to its high-energy state, releasing the myosin head from the actin active site.